We have seen that in few of the cases the registration token is not getting added. These normally test subscribers. Please let me know your suggestions to identify the issue.
First, you should check, what response mobile app receiving when registering these users. It might be possible, these request has not been reached to adobe server.
Error code gives you some more information.
Make sure all the required data for registerMobile function should be delivered to adobe campaign server. It might be there is some condition of this function is not satisfied.
You can send a request manfully with Postman to the adobe campaign server. It will help you to debug it easily.